**Question:** Marker for granulocytic sarcoma:

A. CD34
B. CD117
C. CD33
D. CD13

**Core Concept:** Granulocytic sarcoma is a rare but clinically significant complication of acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). It is a localized tumor formed by immature myeloid cells outside the bone marrow. These cells can infiltrate various organs and tissues, such as skin, lymph nodes, soft tissues, and bones.

**Why the Correct Answer is Right:** Granulocytic sarcoma expresses antigens associated with myeloid lineage cells. In the context of this question, granulocytic sarcoma expresses CD13, a marker for immature granulocytes. CD13 is a type of enzyme called acid phosphatase. Its presence indicates the presence of myeloid cells and helps differentiate granulocytic sarcoma from other infiltrates of mature granulocytes, such as histiocytosis.

**Why Each Wrong Option is Incorrect:**

A. CD34 (Option A): CD34 is a common marker for hematopoietic stem cells and early hematopoietic progenitor cells. However, it is not specific to immature granulocytes, making it an incorrect marker for granulocytic sarcoma.

B. CD117 (Option B): CD117 is a receptor tyrosine kinase, mainly associated with CML, not granulocytic sarcoma. While it is expressed in some cases of granulocytic sarcoma, it is not specific to immature granulocytes.

C. CD34 (Option C): As explained above, CD34 is not specific to immature granulocytes and is not a suitable marker for granulocytic sarcoma.

D. CD13 (Correct Answer): CD13 is a type of enzyme called acid phosphatase found on immature granulocytes. Its expression is specific to granulocytic sarcoma, making it the correct marker for this condition.

**Clinical Pearl:** Granulocytic sarcoma is a rare but clinically significant condition, often presenting as a localized tumor outside the bone marrow in patients with AML or CML. A thorough clinical history, physical examination, and appropriate investigations are essential to diagnose granulocytic sarcoma.

**Correct Answer Explanation:** Granulocytic sarcoma is a rare extramedullary tumor resulting from the infiltration of leukemic granulocytes outside the bone marrow. The correct marker for granulocytic sarcoma is CD13, a marker for immature granulocytes and an enzyme called acid phosphatase.

**Why Each Wrong Option is Incorrect:**

A. CD34 is a marker for hematopoietic stem cells and early hematopoietic progenitor cells. It is not specific to granulocytes and is not an ideal marker for distinguishing granulocytic sarcoma.

B. CD117 is a tyrosine kinase receptor that is mainly associated with CML and is not specific to granulocytes. It is not an ideal marker for diagnosing granulocytic sar